In active matrix displays (AM-LCD) having an integrated non-linear device as a switching device typified by a non-linear two terminal device such as a variable resistor or diode, and a non-linear three terminals device such as a thin film transistor (TFT), liquid crystal compositions having a high voltage holding ratio are required compared with conventional displays of passive modes such as a twisted nematic mode, guest host mode, super twisted nematic mode, and SBE (Super Birefringence Effect) mode. AM-LCD utilizing a liquid crystal composition having a low voltage holding ratio can not provide a high quality display, since it causes phenomena such as spotting, flickering, and lowering in contrast. Besides, the lowering of voltage holding ratio under the environment in which an AM-LCD is used raises other problems.
Since, in many instances, liquid crystal materials, as a single compound, used for displays do not satisfy various requirements for properties such as temperature range of liquid crystal phase, viscosity, optical anisotropy, dielectric anisotropy, elastic constants (K.sub.11, K.sub.22, K.sub.33), driving voltage, chemical and physical stability and long term stability of them, consumptive electric current, specific resistance, voltage holding ratio, and temperature dependency of these parameters which meet to various displays, a mixture of a plural number of compounds (hereinafter, sometimes, referred to as liquid crystal composition) is generally used.
Heretofore, it is known that the chemical structure of liquid crystalline compounds used for displays is usually a rod-like, and that the chemical structure can roughly be divided into a terminal group portion, ring structure portion, and the portion of bonding group which links a ring with another ring. Ring bonding group portion may sometimes have lateral substitutents.
As the literature which shows such facts, for example, the following can be mentioned:
Literature 1: Takashi INUKAI, Liquid Crystals-Fundamentals, (co-editors; Kohji OKANO and Shunsuke KOBAYASHI), pp. 178-204, published by Baifuhkan Co., Ltd. (1985) PA1 Literature 2: Shunsuke TAKENAKA, Liquid Crystal Materials (edited by Narikazu KUSABAYASHI), pp. 67-94, published by Kohdansha Co., Ltd. (1991) PA1 Literature 3: V. Vill, Landolt-Boernstein/New Series Group IV Volume 7 a-d Liquid Crystals, Springer-Verlag (1992) PA1 Literature 4: D. Demus, H. Demus, H. Zaschke, Fluessige Kristalle in Tabellen, VEB Deutscher Verlag fuer Grundstoff Industrie, Leipzig (1976) PA1 Literature 5: D. Demus, H. Zaschke, Fluessige Kristalle in Tabellen II, VEB Deutscher Verlag fuer Grundstoff Industrie, Leipzig (1984), PA1 (1) A liquid crystal composition comprising at least one liquid crystalline compound expressed by the following general formula (3) and having a total number of carbon atoms of 4 to 24 in the portions of acyclic hydrocarbon in a side chain, in an amount of 70% by weight or more, in the liquid crystal composition, and the average value of the total number of carbon atoms in the liquid crystalline compound in the liquid crystal composition is 5 or more. ##STR1## wherein R represents a chain group having 1 to 12 carbon atoms in the portion of hydrocarbon, A1 and A2 independently represent trans-1,4-cyclohexylene group, or 1,4-phenylene group which may be unsubstituted or lateral position of which may be substituted with fluorine, Z1 and Z2 independently represent single bond, or a bridge group a main chain of which is composed of an even number of atoms, X represents --F, --Cl, --CF.sub.3, --OCF.sub.3, --OCHF.sub.2, --OC.sub.2 F.sub.5, or--OCF.sub.2 CHF.sub.2, Y1 represents flourine atom, Y2 represents hydrogen atom or fluorine atom, and m is 0, 1, 2, or 3. PA1 (2) The liquid crystal composition recited in the aspect (1) mentioned above wherein the portions of the acyclic hydrocarbon are an alkyl group or alkoxy group. PA1 (3) A liquid crystal display utilizing a liquid crystal composition recited in either aspect (1) or (2) mentioned above. PA1 (4) A liquid crystal display of active matrix mode utilizing a liquid crystal composition recited in either aspect (1) or (2) mentioned above. PA1 (5) A liquid crystal composition comprising at least one liquid crystalline compound represented by the following general formula (1) and having a total number of carbon atoms of 4 to 24 in the portions of acyclic hydrocarbon in a side chain, in an amount of 70% by weight or more in the liquid crystal composition, and the average value of the total number of carbon atoms in the liquid crystalline compound in the liquid crystal composition is 5 or more ##STR2## wherein R1 represents an alkyl group or alkoxy group having 4 to 24 carbon atoms; A, B, and C independently represent 1,4-phenylene, trans-1,4-cyclohexylene, 1,3-dioxane-2,5-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl provided that when A, B, and C are 1,4-phenylene, 1 to 4 hydrogen atoms of which may independently be substituted with F, Cl, OCF.sub.3, or CF.sub.3 ; a, b, and c are independently an integer of 0 to 3 provided that a+b.+-.c.ltoreq.3; Z1 and Z2 independently represent single bond, --COO--, or --C.tbd.C--; X represents F, Cl, O(CH.sub.2).sub.n CH.sub.l F.sub.3-l, (CH.sub.2).sub.n CH.sub.l F.sub.3-l, or an alkyl or alkyloxy group having 1 to 24 carbon atoms provided that n is an integer of 0 to 4, and l is an integer of 0 to 2; and Y1 represents for Cl an Y2. PA1 (6) The liquid crystal composition recited in the aspect (5) mentioned above wherein the liquid crystal composition has a CN point or SN point of -10.degree. C. or lower, and a NI point of 60.degree. C. or higher, and is composed of at least 7 liquid crystalline compounds, in which CN point represents crystal-nematic phase transition point, SN point represents smectic-nematic phase transition point, and NI point represents nematic-isotropic liquid phase transition point. PA1 (7) A liquid crystal display utilizing a liquid crystal composition recited in either aspect (5) or (6) mentioned above. PA1 (8) A liquid crystal display of active matrix mode utilizing a liquid crystal composition recited in either aspect (5) or (6). PA1 (9) A process for producing a liquid crystal composition comprising selecting the liquid crystalline compounds mentioned above and their content by using, as a parameter, the number of carbon atoms in the portion of acyclic hydrocarbon in liquid crystalline compounds from which the liquid crystal composition is composed, within the range wherein the total number of carbon atoms in the portions of acyclic hydrocarbon in the liquid crystalline compounds is from 4 to 24, and the average value of the total number of carbon atoms in the portions of acyclic hydrocarbon in the liquid crystalline compounds in the liquid crystal composition is 5 or more, so that the voltage holding ratio of a cell of the liquid crystal is a prescribed value or higher when the cell was prepared by filling the liquid crystal composition. PA1 (10) The process for producing a liquid crystal composition recited in the aspect (9) mentioned above wherein the liquid crystalline compounds are selected from the compounds expressed by the following general formula (4), and its content is 70% by weight or more ##STR3## wherein R1 represents an alkyl group or alkoxy group having 4 to 24 carbon atoms, A, B, and C independently represent 1,4-phenylene, trans-1,4-cyclohexylene, 1,3-dioxane-2,5-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl group, when A, B, and C are 1,4-phenylene, 1 to 4 hydrogen atoms in the phenylene may independently be replaced by F, Cl, OCF.sub.3, or CF.sub.3, a, b, and c are independently an integer of from 0 to 3 provided that a+b+c.ltoreq.3, Z1 and Z2 independently represent single bond, --COO--, --CH.sub.2 CH.sub.2 --, or --C.tbd.C--, X represents F, Cl, O(CH.sub.2).sub.n CH.sub.l F.sub.3-l, (CH.sub.2).sub.n CH.sub.l F.sub.3-l, or an alkyl group or alkyloxy group having 1 to 24 carbon atoms wherein n is an integer of from 0 to 4 and l is an integer of from 0 to 2, and Y1 and Y2 independently represent H, F, or Cl. PA1 (11) A liquid crystal composition comprising 70% by weight or more in total of seven or more kinds of the liquid crystalline compounds expressed by the following general formula (3) and having a total number of carbon atoms in a side chain of 4 to 24, average value of the total number of the carbon atoms in the liquid crystalline compounds in the liquid crystal composition being 5 or more ##STR4## wherein R represents a chain group having 1 to 12 carbon atoms in the portions of hydrocarbon, A1 and A2 independently represent trans-1,4-cyclohexylene group or 1,4-phenylene group which phenylene group may be unsubstituted or lateral portion of which may be substituted with fluorine atom, Z1 and Z2 independently represent covalent bond or a bridge group main chain of which is composed of an even number of atoms, X represents --F, --Cl,, --CF.sub.3, --OCF.sub.3, --OCHF.sub.2, --OC.sub.2 F.sub.5, or --OCF.sub.2 CHF.sub.2, Y1 represents fluorine atom, Y2 represents hydrogen atom or fluorine atom, and m is 0, 1, 2, or 3, provided that the bridge group does not include --(CH.sub.2 CH.sub.2).sub.P -- wherein p is 1 or 2.
In more specific explanation, liquid crystalline compounds used for displays are organic compounds having the structure expressed by the following general formula (2) EQU R--(A.sup.1 --Z).sub.n --A.sup.2 --X (2)
wherein R represents a group in a chain form, A.sup.1 and A.sup.2 represent a group in a ring form, Z represents covalent bond or a bonding group (bridge), n is an integer of 1 or greater, and X represents an electron attractive group or a group in a chain form.
Generally, R is called "side chain", the portion of --(A.sup.1 --Z).sub.n -- A.sup.2 -- is called "core", and X is called "terminal group". When the terminal group X is an electron attractive group, it is called "polar group", and when this is a chain form group, it is called another "side chain". In some cases, the ring of the "core" has some substitutents such as halogen at its side position, and such substituents are called "lateral group". While n is an integer of 1 or greater, liquid crystalline compounds in which n is 4 or greater are rare. Also, when n is 2 or greater, in many cases, (A.sup.1 --Z) is not the repetition of the same structure.
As the side chain corresponding to R and X, an alkyl group, alkoxy group, alkenyl group, alkenyloxy group, and oxalkyl group are typical ones. Whereas these groups are usually straight chains, sometimes they are branched. As the polar group corresponding to X, cyano group, a halogen, --CF.sub.3, --OCF.sub.3, --OCHF.sub.2, --OC.sub.2 F.sub.5, --OCF.sub.2 CHF.sub.2, and --NCO are typical ones.
As the ring form group A.sup.1 and A.sup.2 in the core, while six-membered rings such as 1,4-phenylene, trans-1,4-cyclohexylene, 1,3-dioxane-trans-2,5-diyl, and pyrimidine-2,5-diyl are typical ones, five-membered rings, four-membered rings, and condensed rings such as naphthalene are also known. In the case of aromatic rings such as 1,4-phenylene and pyrimidine-2,5-diyl, hydrogen atoms at a side position may sometimes be substituted with a polar group such as a halogen.
As the case in which Z is a bridge group, groups in which a main chain is composed of an even number of atoms, such as --CH.sub.2 CH.sub.2 --, --CH.sub.2 O--, --OCH.sub.2 --, --COO--, --OCO--, --CF.sub.2 O--, --OCF.sub.2 --, and --(CH.sub.2).sub.4 -- are known.
Also, there is a case in which hydrogen atom in the compound is substituted with an isotope such as deuterium atom.
Liquid crystalline compounds are classified by their value of dielectric anisotropy (.DELTA..di-elect cons.), and the compounds having a positive value of .DELTA..di-elect cons. are called P type and those having a negative or small value of .DELTA..di-elect cons. are called N type. Compounds expressed by general formula (2) in which X is a polar group are P type, and the compounds in which X is a side chain are N type.
Accordingly, the molecular structure of liquid crystalline compounds can be designed by combining various partial structures, and liquid crystal compositions have been designed by selecting and mixing compounds according to the purpose of their use.
In a conventional structure design of liquid crystalline compounds, the viewpoint considered to be most important is the structure of the polar group and core which strongly affect the physical properties of liquid crystal materials for displays. In AM-LCD, elevation of voltage holding ratio has been required, and one of the physical properties which affects the voltage holding ratio is considered to be specific resistance. That is, liquid crystal materials having a high specific resistance are considered to be necessary for AM-LCD. It is known that as liquid crystal materials to be used for AM-LCD, the materials comprising the compounds having --F, --Cl, --OCF.sub.3, --OCHF.sub.2, --CF.sub.3, --OCF.sub.2 CHF.sub.2, and --OC.sub.2 F.sub.5 as polar group or lateral groups are preferable by their higher specific resistance than the materials comprising the compounds having cyano group. Accordingly, methods for elevating specific resistance, by paying attention to electron attractive groups, have been applied in prior art. However, the prior art wherein attention is directed to electron attractive groups is unsatisfactory in obtaining liquid crystal compositions suitable for AM-LCD having a higher voltage holding ratio.